In many types of electronic devices and equipment, such as calculators, computers, CRT terminals, pushbutton telephones and the like, all hereinafter generally referred to simply as electronic apparatus, there has been an ever-increasing need for circuit-controlled keyboard switch modules of the type wherein each key (or pushbutton) is capable of initiating or controlling multiple work (or logic) functions performed by the apparatus. With the advent of large-scale integrated circuits, the above types of keyboard-operated apparatus, cited by way of example only, has progressively become more complex, and versatile, while at the same time allowing for the ever smaller packaging thereof, with the often attendant need for smaller keyboards. The emphasis on miniaturization, of course, has also advantageously generally resulted in a reduction in the costs of manufacturing such apparatus and, in many cases, lead to an improvement in the esthetics thereof, as packaged.
As a result of the continuously increasing work function capabilities built into diverse types of keyboard-controlled electronic apparatus, a serious problem has arisen wherein the number of work (or logic) functions that are capable of being performed therewith are often limited by the actual number of key (or pushbutton) type switches that can be practically placed on the associated housing-mounted keyboard. Compounding this problem, of course, is the fact that in any given keyboard, the keys (or pushbuttons) must be sufficiently spaced, and of adequate size, so as to allow an operator to conveniently and reliably actuate them rapidly and in any selected order.
One technique employed heretofore to allow each keyboard switch to selectively initiate or control two different work (or logic) functions has been to use a so-called "shift" or "prefix" key. Disadvantageously, such shift or prefix keys are in addition employed to actually initiate or control a specified work function and, hence, require additional, and normally high priority, space on a given keyboard. In addition, such shift or prefix keys, whether responsively connected electronically or mechanically (e.g., by a shuttle plate) to all or selected ones of the regular work function key (or pushbutton) switches, requires multiple key actuations (e.g., multiple button depressions) by an operator in order to initiate different predetermined work (or logic) functions per switch. This can readily contribute to operator fatigue and reduce efficiency, particularly over long operator work periods.